Project Summary: The main goal of this project is to characterize a new therapeutic target in order to improve breast cancer therapy in preventing breast cancer from spreading and restoring the effectiveness of hormone therapy. Despite most breast cancers are diagnosed during relatively early stage, nearly 30% of them will eventually develop metastasis in spite of treatment. The key factors that drive breast cancer metastasis remain to be discovered. On the other hand, while estrogen receptor (ER) is the most successful therapeutic target in breast cancer, up to one-third of breast cancers lose ER expression, thus do not respond to endocrine therapy. The mechanisms for ER loss in the majority of ER-negative breast cancers also remain to be investigated. We have now identified a signaling molecule 14-3-3? as a key driver that promotes breast cancer metastasis and ER loss. We propose (1) to use 14-3-3? to model human breast cancer progression, (2) to determine a role for ER?36 in 14-3-3?-promoted ER loss in breast cancer, and (3) to utilize the animal model that we have established to test the inhibitors targeting the 14-3-3? pathways to prevent breast cancer metastasis and restore the response to hormone therapy. Some small molecule inhibitors for the proposed pathways have been available in clinics or been tested in clinical trials for other conditions. Thus, if confirmed, it would be quite feasible to test them in patients with tumors harboring high levels of 14-3-3?. Through the examination of the 14-3-3? expression in the breast tumor samples, we might be able to identify the patients who are at risk of developing metastasis and losing response to endocrine therapy. These patients may benefit from treatment with these inhibitors targeting the downstream effectors of 14-3-3? to prevent breast cancer metastasis. The potential impact of this proposal in providing a novel therapeutic strategy to prevent breast cancer metastasis and to restore endocrine sensitivity in ER(-) breast cancer is very significant.